1. Field of the Invention
This invention pertains generally to processes for the multi-color printing and decorating of formed articles such as pre-formed containers.
2. Description of the Prior Art
Several well-known processes are available for single color printing of articles and containers, including such standard methods as silk screening, direct letterpress printing, transfer applications and offset printing. Direct letterpress printing and conventional lithographic and dry offset printing are particularly useful for the high speed printing of paper webs and sheets which can be moved flat through the letterpress printing press or between the blanket cylinder and impression cylinder of a conventional offset printing press. Multiple colors may be printed on the sheet or web, but this is usually done by applying each color separately to the paper with separate letterpress plates, or with separate blanket cylinders where an offset press is employed.
While the conventional multi-color offset printing process is adequate for the printing of paper webs, it is not well adapted to the printing of formed containers and other articles which cannot be passed flat between the blanket and impression cylinders of the offset press. Examples of such containers and articles are blow molded, thermoformed, or extruded plastic containers, metal cans which are drawn or otherwise formed from a blank, spiral wound composite containers, and glass bottles. Applying multiple ink films to such formed containers by using a plurality of blanket cylinders, each having a separate ink color, is generally not a practical method because of the difficulty of precisely aligning the multiple colored patterns on the article being printed. The necessity of precise registration of the color patterns greatly slows the handling of the articles, while the variations often encountered in the physical dimensions of the articles will result in a high percentage of poorly decorated articles. Similar problems are encountered where multi-color direct letterpress printing of formed articles is attempted.
Other common types of article decorating processes such as silk screening and the application of labels or transfers are excessively expensive for multi-color decoration and often do not provide adequate decoration quality.
Another approach to the multi-color printing of formed containers involves the application of multiple films of ink onto a carrier blanket on a single blanket cylinder by the dry offset printing process, such that each layer of ink will not be in contact with a layer of ink which is previously or subsequently applied to the blanket. The rubber carrier blanket on the blanket cylinder, with the ink films thereon, may then be placed in printing contact with the article to be printed, with a resulting transfer of the ink films from the blanket surface to the article surface. These ink films will split during transfer, leaving residual ink films on the rubber carrier blanket. Thus, these ink films applied to the blanket must be separated, since if they come in contact with each other, one ink color may be picked up by the plate which prints another ink color onto the blanket. This will result in inks being mixed and distortion of colors. Moreover, if the inks applied to the blanket are fairly thin and have low cohesion, a film of a second ink applied over a first film of ink may split the first film or mix with it. Because the ink films are separated, the resulting printed work does not have the desired degree of sharpness and clarity. In addition, such a technique is obviously not capable of printing multi-color overlapped line images or multi-color halftones.
Attempts have been made to avoid these problems associated with the printing of overlapped colors by selecting the cohesiveness of each layer of ink film such that the films will overlay one another without splitting previous or subsequent layers. This technique does not totally avoid the problem of ink colors migrating onto the printing plate of a different ink color, and it requires complicated printing procedures. Since the multiple layers of ink films have varying cohesive values, each layer of ink film may not split to the same degree upon transfer to the surface being printed. This results in an excess transfer of some ink colors, and an insufficient transfer of other colors.